1. Field of the Invention
The present invention relates to boronized sliding material.
More particularly, the present invention relates to boronized sliding material which is highly resistant to seizure by the opposite material when sliding.
2. Description of Related Arts
Boronizing is broadly employed for surface treatment for enhancing the wear resistance of mainly ferrous materials.
Borides having an ultra-high hardness are formed on the surface of boronized ferrous materials and make them more wear-resistant. The presence of FeB and Fe.sub.2 B are shown in the Fe-B equilibrium phase diagram. It is these borides that are formed in the surface layer of boronized materials. Hardness of FeB ranges from Hv 1800 to 2000, and hardness of Fe.sub.2 B ranges from Hv 1400 to 1800. The phase diagram shows that the structure of boronized ferrous material is a single FeB phase, dual mixed phase of FeB and Fe.sub.2 B, or a single Fe.sub.2 B phase. In most of the boronized materials, however, the structure is the dual mixed FeB and Fe.sub.2 B phases. This is because: FeB, which is brittle, is not appropriate, as a single surface phase, for the sliding surface, and, the quantity of boron-impregnation for obtaining the single FeB phase is difficult; and, further the single FeB phase does not exhibit a high seizure resistance, particularly under conditions where the oil supply is liable to be interrupted.
The cross section of the sliding surface composed of dual mixed FeB and Fe.sub.2 B phases exhibits minute uneveness which is formed by protruding FeB and recessing Fe.sub.2 b due to the difference of hardness between FeB and Fe.sub.2 B. Since FeB is hard and brittle. The protrusions occasionally break during sliding and the broken fragments damage the opposite member, and cause sudden wear. When the opposite member wears drastically and the lubrication is severe, as described above, the opposite member softens and then the fusion bonding is liable to occur. The seizure resistance of the conventionally boronized, ferrous sliding materials is not said to be satisfactory, as is described above.
It has long been considered that the phases present in the Fe-B system are FeB, Fe.sub.2 B and Fe. Formation of Fe.sub.3 B phase has, however, relatively recently been discovered in the formation of iron borides by CVD (chemical vapor deposition).